Subsurface Modifier Principle
There are no rules.
  Use what works for you :).




Sometimes bevel doesn't give good results, so subdivision needs to be used instead.
They can also be used together for easier cotrol.

Bevel modifier - easily adjustable edges, limited in tight corners (creates overlapping geometry)
Subdivision Modifier - works almost anywhere because it actually moves the geometry.

Using Bevel and Subdivision together